Spark plasma sintering of amorphous-crystalline laminated composites

Materials Science and Engineering A 528 (2011) 1901–1905

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Rapid communication

Spark plasma sintering of amorphous-crystalline laminated composites Sandip P. Harimkar ∗ , Tushar Borkar, Ashish Singh School of Mechanical and Aerospace Engineering, Oklahoma State University, 218 Engineering North, Stillwater, OK 74078, United States

a r t i c l e

i n f o

Article history: Received 10 August 2010 Received in revised form 11 October 2010 Accepted 30 October 2010

Keywords: Bulk amorphous alloys Composites Sintering Phase transformation X-ray diffraction

a b s t r a c t In this rapid communication, a novel approach involving pulsed electrodeposition (PED) and spark plasma sintering (SPS) of Fe–B–Si amorphous ribbons is presented for the processing of amorphous-crystalline laminated composites. The PED allowed the deposition of nanocrystalline Ni or Cu layers (thickness: <3 ␮m) on the Fe–B–Si amorphous ribbons (thickness: <25 ␮m). The coated ribbons were subsequently stacked and SPS sintered under the influence of pulsed direct current and uniaxial pressure. The SPS sintered amorphous ribbons (uncoated) and amorphous-Ni laminates exhibited partial crystallization of the ribbons and poor lamellar bonding in the composites. With the similar SPS sintering parameters (temperature: 450 ◦ C; pressure: 350 MPa; sintering time: 20 min), the amorphous-Ni–Cu laminates were sintered to full densification without undesirable crystallization of the amorphous ribbons. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Amorphous materials or bulk metallic glasses (BMG) represent a new class of advanced materials exhibiting an attractive combination of properties, such as high strength (up to 4 GPa), elastic limit (up to 2–3%), and excellent wear/corrosion resistance important for structural applications [1–4]. These outstanding properties are primarily due to disordered atomic arrangement in the amorphous materials resulting in absence of grain boundaries and defects in the microstructure. Despite these outstanding properties, utilization of amorphous materials in structural applications has been limited primarily due to difficulties in fabrication of large-size (bulk) products using conventional casting technologies and their nearcomplete brittleness (limited/no plastic deformation) [5]. Due to requirement of rapid cooling rates (up to 103 K/s) to achieve amorphous structure, only small castings (diameters <1 cm), thin foils (thickness <100 ␮m), and powders (diameter <100 ␮m) can typically be fabricated using conventional solidification processing (casting, melt spinning, and gas atomization) [6–11]. While significant progress has been made in increasing the critical diameter of the amorphous castings, this has limited the amorphous compositions to certain multi-component systems with enhanced glass forming abilities. Since powder and thin foils of amorphous material can be readily produced, several attempts have been made to consolidate them into bulk shapes using solid state sintering (hot pressing) [12,13]. For example, Zhou et al. investigated consolidation behavior of Ni-based amorphous ribbons [14]. The study

∗ Corresponding author. Tel.: +1 405 744 5900; fax: +1 405 744 7873. E-mail address: [email protected] (S.P. Harimkar). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.107

concluded that very high pressures (>1.5 GPa) and longer sintering times (>1.5 h) at temperatures close to crystallization temperature of the alloy are required to consolidate near-fully dense (>90% relative density) amorphous samples. Further increase in the sintering temperature above crystallization temperature caused crystallization of the samples. While conventional powder metallurgical processes have showed some promise for the bulk processing of amorphous materials, it is still difficult to achieve full densification of amorphous powder without crystallization. The other challenge restricting the utilization of amorphous alloys for structural applications is related to their near-complete brittleness accompanied with strain softening and shear localization. It is now well accepted that the catastrophic failures of bulk metallic glasses are driven primarily by the initiation and propagation of localized regions of extensive plastic deformation known as shear bands [15–18]. Significant progress has been made in enhancing the global plasticity of the bulk amorphous alloys by introducing or forming crystalline phases in amorphous matrices, and thus forming amorphous matrix composites [19–21]. The reinforced crystalline phases promote the shear band initiation and also hinder the propagation of shear bands, thus enhancing the global plasticity of amorphous materials. Since most of these composites were formed during conventional solidification processing, the challenges related to bulk processing still remains. To overcome the difficulties in bulk processing and enhance the plasticity of the amorphous materials, several attempts have been made to fabricate composite structures with alternating layers of starting amorphous ribbons and soft metallic phases. For example, Alphas and Embury reported on fabrication of amorphous Ni78 Si10 B12 copper laminated composites using electrodeposition and diffusion bonding [22]. The copper layers provided constraint to impede the

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Fig. 1. Schematic showing fabrication of amorphous-crystalline laminated composites using pulsed electrodeposition and spark plasma sintering.

propagation of shear bands and enhanced overall ductility of the laminated composites. Leng and Courtney also reported on enhanced ductility of the brass–Ni-based metallic glass laminates fabricated by soldering the constituent layers with a Pb–Sn alloy [23]. While the diffusion bonding and soldering processes demonstrated feasibilities for bulk processing of laminated composites with enhanced ductility, the processes have had limited success in enabling the utilization of amorphous materials for structural applications primarily due to delamination of constituent layers during processing and testing. In this rapid communication, we are presenting a novel approach for fabricating iron-based amorphous-crystalline laminated composites using pulsed electrodeposition (PED) and spark plasma sintering (SPS) processes. Detailed investigations on the phase evolution and interlamellar interfacial characteristics in the amorphous-Ni and amorphousNi–Cu laminated composites are presented. 2. Experimental The schematic of the processing approach is presented in Fig. 1. The iron-based amorphous ribbons having composition Fe85–95 Si5–10 B1–5 (2605 SA1, Metglas Inc.) were used for the present investigation. The reported crystallization temperature of this alloy is 508 ◦ C. The average thickness of the alloy ribbon was 22.86 ␮m. The amorphous ribbons were cut to a size of 70 mm × 25.4 mm and used as substrates (cathode) for pulsed electrodeposition (PED) experiments. In the first set of experiments, crystalline nickel was electrodeposited on amorphous ribbons from nickel sulfate bath (to form amorphous-Ni laminates during subsequent SPS). The average thickness of the nickel coating was 1.75 ␮m. Attempts were also made to electrodeposit copper on the amorphous ribbons. As with any steel substrates, the adhesion of electrodeposited copper on the amorphous ribbon was not adequate. In the second set of experiments, nickel striking was performed for 1 min on the amorphous ribbons to improve adhesion of copper coating (to form amorphous-Ni–Cu laminates during subsequent SPS). The average thickness of the nickel striked layer was 800 nm. The average thickness of subsequent copper coating electrodeposited on the nickel striked amorphous ribbon was 2.2 ␮m. The copper coatings on the nickel striked amorphous ribbons were homogeneous and exhibited very good bonding with the substrate. The elctrodeposition parameters for nickel coatings and copper coatings are presented in Table 1. Spark plasma sintering (SPS) is a relatively new powder metallurgical process and involves simultaneous application of uniaxial pressure and pulsed direct current. The process has previously been

used for sintering nanomaterials. The unique mechanisms involving joule heating at the particle contacts and/or localized spark discharges at the gaps between the particles cause localized surface heating and solid state sintering of nanopowder without significant grain growth. Since the heating effects under the influence of electric current are localized at the powder surface, the SPS process allows sintering within shorter time (within minutes) and at significantly lower temperature compared to conventional hot pressing [24,25]. These attributes also make the process attractive for sintering difficult-to-sinter amorphous materials without undesirable phase transformations [26,27]. In our previous investigations, we have successfully used SPS to sinter amorphous powders into dense compacts. The present investigation is focused on SPS processing of amorphous-crystalline laminates from electrodeposited amorphous ribbons. Following the electrodeposition, the amorphous ribbons were stamped into 12 mm diameter thin foils. The thin foils were then stacked in the tungsten carbide dies and consolidated using SPS. The SPS sintering was conducted using uniaxial pressure of 350 MPa at sintering temperature of 450 ◦ C, which is almost 58 ◦ C below the crystallization temperature of the alloy. A typical sintering cycle consisted of three steps: heating at a rate of 100 ◦ C/min, soaking at temperature of 450 ◦ C for 20 min, and cooling at a rate of 100 ◦ C/min. The final cooling was attained by purging nitrogen into the furnace. To compare the densification behavior, some uncoated amorphous foils were also SPS sintered to produce monolithic bulk amorphous compacts. Following SPS sintering, the monolithic amorphous and laminated composite compacts were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and micro-indentation techniques. The phase analysis of the composites was conducted using Philips Norelco X-ray diffracTable 1 Bath compositions and processing parameters for pulse electrodeposition of copper and/or nickel on Fe–B–Si amorphous ribbons. Electrodeposition parameters

Nickel coating

Copper coatings

Electrolyte

NiSO4 ·6H2 O (265 g/l) NiCl2 ·6H2 O (48 g/l) H3 BO3 (31 g/l) 3

CuSO4 ·5H2 O (200 g/l) H2 SO4 (100 g/l) 3

4.0 25 6 20 10 Nickel

0.25 25 6 20 10 Copper

Peak current density (A/dm2 ) pH Temperature (◦ C) Plating time (min) Duty cycle (%) Frequency (Hz) Anode

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Fig. 2. X-ray diffraction patterns from: (a) starting amorphous ribbon, and spark plasma sintered (b) amorphous ribbons, (c) amorphous-Ni laminates, and (d) amorphous-Ni–Cu laminates (sintered at 450 ◦ C for 20 min with uniaxial pressure of 350 MPa).

˚ radiation at 45 kV tometer operating with Cu K␣ ( = 1.54178 A) and 40 mA. The crystallite size for SPS sintered amorphous ribbons, amorphous-Ni, and amorphous-Ni–Cu laminated composites was calculated using Scherrer equation, given by [28]: FWHM =

K 180 D cos  

(1)

where FWHM is full width half maxima in 2 degrees, D is the crystallite size in nm, K is constant (usually evaluated as 0.94), and  is the wavelength of Cu K␣ radiation. A microhardness tester (Clark CM-700AT) was used for measuring hardness by performing indentations at a load of 2.94 N and holding time of 10 s. The microhardness was measured on the cross-sectional surfaces of the laminates. Around 10 microhardness readings were taken on each surface and an average value is reported. 3. Results and discussion The X-ray diffraction patterns of starting Fe–B–Si amorphous ribbons, SPS sintered ribbons, amorphous-Ni laminates, and amorphous-Ni–Cu laminates are presented in Fig. 2. The presence of broad peak with diffused intensity in the XRD pattern for starting ribbons confirmed that the ribbons used in this investigation had amorphous structure. The XRD pattern of SPS sintered amorphous ribbons (uncoated) exhibited sharp peaks corresponding to ␣-Fe phase superimposed on amorphous background. The ␣-Fe crystallite size measured using Scherrer equation was ∼30 nm. The XRD pattern of amorphous-Ni laminated composites exhibited sev-

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eral constituent nickel peaks and one ␣-Fe peak, suggesting partial crystallization during SPS sintering. The nickel crystallite size in this laminated composite was found to be ∼35 nm. Even though the sintering temperature (450 ◦ C) was almost 58 ◦ C less than the crystallization temperature (508 ◦ C) of the alloy, the partial crystallization of the amorphous ribbons in the compacts was observed. It should be noted that the sintering temperature reported in this study was measured using thermocouple placed inside the die wall during sintering. The actual temperature inside the specimen may be significantly higher than that at the die wall. In view of this, the appearance of superimposed crystalline peaks in XRD pattern of uncoated ribbons and amorphous-Ni composites sintered at temperatures well below the crystallization temperature may be attributed to the local overheating of the ribbon surfaces during sintering. The XRD pattern of the SPS sintered amorphousNi–Cu laminated composites indicated presence of corresponding constituent nickel and copper crystalline peaks. Note that the background broad peak corresponding to amorphous layers is very weak and unresolvable in XRD pattern from amorphous-Ni–Cu laminated composites. In general, the intensity of the broad peak corresponding to amorphous phase is often very weak due to weak X-ray scattering from dis-ordered phase. Furthermore, a small fraction of crystalline phase in the amorphous matrix often diminishes the intensity of the amorphous background. In amorphous-Ni–Cu laminated composites, ∼15% of the cross sectional area corresponds to crystalline phases (thickness of crystalline layer: ∼3 ␮m; thickness of amorphous layer: ∼22 ␮m). At such a large volume fraction of strongly diffracting crystalline phases, the absence of background intensity corresponding to amorphous layers is not surprising. The absence of peaks corresponding to crystallization products (␣-Fe and Fe2 B) of amorphous ribbon indicates that the starting glassy structure of the ribbons was retained in the SPS sintered amorphous-Ni–Cu laminated composites. The nickel and copper crystallite sizes for this laminated composite were 56 and 45 nm, respectively. Interestingly, no evidence of crystallization of amorphous ribbons was observed in this case of laminated composites, SPS sintered using same processing parameters as with the uncoated ribbons and amorphous-Ni laminates. It seems that the presence of high conductivity constituent copper in the amorphous-Ni–Cu laminated composites efficiently conducts the heat away from the ribbon surfaces resulting in shallow temperature gradient. Thus, actual temperature inside the specimen could be close to or slightly higher than the measured temperature from the die wall (450 ◦ C). Since this temperature is still less than the crystallization temperature of the alloy, devitrification of the amorphous ribbons was not observed during SPS sintering of

Fig. 3. Cross-sectional SEM micrographs from spark plasma sintered: (a) amorphous ribbons, (b) amorphous-Ni laminates, and (c) amorphous-Ni–Cu laminates sintered at 450 ◦ C with uniaxial pressure of 350 MPa.

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Fig. 4. Microindentations at/near the interfacial regions of (a) amorphous-Ni and (b) amorphous-Ni–Cu laminated composites.

this laminated composite. Significant experimental and computational efforts need to be directed towards understanding the extent of mismatch between actual specimen temperature and measured temperature from the die wall while taking into consideration important factors like thermophysical properties of the constituent materials, size of die, insulation of the die, and vacuum level. The microstructures in the cross-sections of the SPS sintered amorphous ribbons, amorphous-Ni, and amorphous-Ni–Cu laminates are presented in Fig. 3. Clearly, the uncoated amorphous ribbons exhibited fracture and complete debonding when SPS sintered with sintering temperature of 450 ◦ C and uniaxial pressure of 350 MPa (Fig. 3a). The amorphous-Ni laminates showed some

regions of good bonding and some regions of debonding (Fig. 3b). It may be noted that both the samples (amorphous ribbons and amorphous-Ni laminates) underwent partial crystallization during SPS (see Fig. 2). On the other hand, amorphous-Ni–Cu laminates, in which amorphous structure of the ribbons was retained, exhibited very good bonding and attained full densification (Fig. 3c). To get better understanding about the cracking/debonding tendency of the laminated composites, Vickers micro-indentations were performed with a vertical force of 2.94 N at/near the interfacial regions of the laminates (Fig. 4). The micro-indentation induced severe cracking along the interface indicating weak interfacial strength for amorphous-Ni laminates. In case of the amorphous-Ni–Cu lam-

Fig. 5. SEM/EDS elemental mapping in the interfacial region of amorphous-Ni–Cu laminates.

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inates, the interface was intact following the micro-indentation suggesting very good interfacial strength. To understand the interface characteristics of the amorphous-Ni–Cu laminates, elemental mapping in the interfacial region was performed using SEM/EDS (Fig. 5). The figure clearly shows distinct layers of nickel (∼800 nm) and copper (∼2.2 ␮m) without any elemental overlapping suggesting formation of strong metallurgical bonding. Furthermore, the EDS elemental maps show the uniform distribution of Fe and B in the amorphous layers indicating retaining of bulk amorphous structure in the laminate without crystallization. The microhardness of the amorphous-Ni laminated composites was found to be 770 ± 50 HV which is significantly less than the hardness of the as-received amorphous ribbons (∼900 HV). Note that these samples showed partial crystallization upon sintering with several regions of interlamellar debonding. Since Fe–B–Si ribbons generally undergo crystallization induced embrittlement and associated increase in hardness, the decrease in the hardness in the present case seems to be due to delamination and interfacial cracking upon indentation rather than partial crystallization [29]. The hardness of amorphous-Ni–Cu laminated composites was found to be 890 ± 50 HV, which is close to the hardness of the as-received amorphous ribbons (∼900 HV). The consistent hardness of these composites seems to be due to good interlamellar bonding and retaining of the amorphous structure of the ribbons. 4. Conclusions Pulsed electrodeposition (PED) and spark plasma sintering (SPS) can be successfully used to fabricate bulk Fe–B–Si amorphouscrystalline laminated composites. The SPS sintered amorphous ribbons and amorphous-Ni laminates exhibited partial crystallization of the ribbons and poor lamellar bonding in the composites. With the similar SPS sintering parameters (temperature: 450 ◦ C; pressure: 350 MPa; sintering time: 20 min), the amorphous-Ni–Cu laminates were sintered to full densification without undesirable crystallization of the amorphous ribbons. While the PED/SPS approach appears promising for the processing of bulk amorphouscrystalline laminates, further understanding of the fundamental mechanisms of sintering taking into account the deformation

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behaviors and the thermal effects is needed to significantly advance the state-of-the art. Acknowledgements This work was supported in part by the U.S. National Science Foundation (Award No. 0969255) and start-up funds from the Oklahoma State University, Stillwater. We thank Dr. Jim Puckette for the use of his X-ray diffraction facilities. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

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